Idealism states that mind or consciousness constitutes the fundamental reality, or is primary. Some versions of idealism admit the existence of material objects, others deny that material objects exist independently of human perception.
Anaximander (Greek philosopher, c. 611 BC - c. 547 BC) may have been the first idealist philosopher. Only one fragment of his writing has been preserved, but he seems to have thought that the original and primary substance (which could be mind) is a boundless something from which all things arise and to which they all return. He was struck by the fact that the world presents us with a series of opposites, of which the most primary are hot and cold, wet and dry. He thought of these opposites as being "separated out" from a substance which was originally undifferentiated.
Plato (Greek philosopher, c. 428 BC - c. 348 BC) is often considered the first idealist philosopher, chiefly because of his metaphysical doctrine of Forms. Plato considered the universal Idea or Form, sometimes called an archetype--for example, redness or goodness--more real than a particular expression of the form--a red object or a good deed. According to Plato, the world of changing experience is unreal, and the Idea or Form--which does not change and which can be known only by reason--constitutes true reality. Plato did not recognize mystical experience as a route to true reality, only reason.
Idealism was first expounded by Plato in his cave allegory in The Republic (c. 360 BC) (see, e.g., Julia Annas, An Introduction to Plato’s Republic,1981, p. 252). The cave is a metaphor for the mind. Prisoners are in an underground cave with a fire behind them, bound so they can see only the shadows on the wall in front of them, cast by puppets manipulated behind them. They think that this is all there is to see; if released from their bonds and forced to turn around to the fire and the puppets, they become bewildered and are happier left in their original state. They are even angry with anyone who tries to tell them how pitiful their position is. Only a few can bear to realize that the shadows are only shadows cast by the puppets; and they begin the journey of liberation that leads past the fire and right out of the cave into the real world. At first they are dazzled there, and can bear to see real objects only in reflection and indirectly, but then they can look at them directly in the light of the sun, and can even look at the sun itself.
This allegory is related to idealism in the following way. The cave is the mind. The shadows of the puppets that the prisoners are watching represent their taking over, in unreflective fashion, the second-hand opinions and beliefs that are given to them by parents, society, and religion. The puppets themselves represent the mechanical, unreasoning minds of the prisoners. The light of the fire within the cave provides only partial, distorted illumination from the imprisoned intellects. Liberation begins when the few who turn around get up and go out of the cave. Outside of the cave, the real objects (the Forms) are those in the transcendental realm. In order to see them, the light of the sun, which represents pure reason, is necessary. A similar allegory using today’s symbols would replace the cave with a movie theater, the shadows with the pictures on the screen, the puppets with the film, and the fire with the projector light. The sun is outside, and we must leave the theater to see its light (we must leave the mind).
The eighteenth century British philosopher George Berkeley (1685 - 1753) was one of the major exponents of idealism. He denied the existence of material substance (calling his philosophy immaterialism), and held that the universe consists of God, which is the infinite spirit; of finite spirits including human beings; of ideas that exist only in the minds of spirits; and of nothing else. His most characteristic philosophical doctrine is summarized in the expression "to be is to be perceived." In other words, to say that a material object exists is to say that it is seen, heard, or otherwise perceived by a mind. Since Berkeley assumed that material objects exist without human minds to perceive them, the mind that perceives them must be divine rather than human.
The German philosopher Immanuel Kant (1724 - 1804) expounded a form of idealism that he called transcendental idealism. He believed that there is a reality that is independent of human minds (the noumenon, or thing-in-itself), but that is forever unknowable to us. All of our experience, including the experience of our empirical selves (the phenomenon, or thing-as-it-appears), depends on the activity of a transcendental self, also of which we can know nothing.
Georg Wilhelm Friedrich Hegel, also a German philosopher (1770 - 1831), built on the idealist philosophy of Kant, and called his system absolute idealism. He believed that reality is Absolute Mind, Reason, or Spirit. This Mind is universal, while each individual mind is an aspect of it, as is the consciousness and rational activity of each person. Absolute Mind continually develops itself in its quest for its own unification and actualization. For this purpose, it manifests itself as the subjective consciousness of the individual, who undergoes a rational process of development from a purely materialistic and self-centered state to a universal and rational consciousness. In this process, the individual passes through several phases--family, society, state--each of which represents a move from individualism to unity. Human history in general is the progressive move from bondage to freedom. Such freedom is achieved only as the separate desires of the individual are overcome and integrated into the unified system of the state, in which the will of the individual is replaced by the will of all.
The forms of idealism described above were all formulated by Western philosophers, who almost exclusively depended on rational thought to develop their philosophies. They scarcely took account of the many forms of Eastern philosophy, which are heavily dependent on mystical experience. Furthermore, there was very little recognition of the theories and knowledge that science was developing from the 17th century on.